We have employed ab initio molecular dynamics simulations to study the oxidation behavior of TiAlN hard coatings as a function of Al content and temperature. Results show that for TiAlN with a low Al content (Ti _0.75 Al _0.25 N), Ti atoms can always bond with O atoms, while Al atoms bond with O only at a higher temperature. For Ti _0.5 Al _0.5 N, both Al and Ti can bond with O atoms, irrespective of temperature. Through analyzing the displacement height of O-bonded metal atoms, we suggest that titanium oxide nucleates at the outermost layer of Ti _0.75 Al _0.25 N while the outermost layer after Ti _0.5 Al _0.5 N is exposed to oxygen is aluminum oxide. Our simulation results predict, in agreement with experiment, that Ti _0.5 Al _0.5 N has superior oxidation resistance in comparison with Ti _0.75 Al _0.25 N. This study provides an atomistic insight to the initial stage of the oxidation process, which is else difficult to observe experimentally.